First we have to define enzymes. Enzymes are protein catalysts with specificity for both reaction catalyzed and its substrates. They can speed up cellular reactions and lower the energy of activation (which is necessary to start a reaction).
There are different ways to classify an enzyme depending on the reaction catalyzed: Oxidoreductase, transferase, hydrolase, lyase, isomerase and ligase. Ligase, as we know join two molecules using ATP (i.e. think of joining DNA strands).